Here it is illustrated how to add RC snubber circuit to CD4047 based inverter circuit. It is explained why snubber circuit is required in inverters, show high voltage spikes generated by transformer and show how by adding RC snubber circuit the voltage spikes can be reduced to safe level.
What is RC snubber circuit?
An RC snubber circuit is a simple but critical protection network used to suppress high-voltage spikes and "ringing" caused by the switching of inductive loads. It consists of a resistor (R) and a capacitor (C) connected in series, typically placed in parallel with a switching component like a MOSFET or across the inductive load itself.
Why is it needed?
In circuits like the inverter, components like transformers or motors act as inductors that store energy in a magnetic field.
1.The Inductive Kick: When a MOSFET (like your IRFZ44N) switches off, the current through the inductor is suddenly interrupted. Since an inductor resists changes in current, it releases its stored energy as a massive voltage spike (V = L times di by dt).
2.Protection: Without a snubber, this spike can exceed the MOSFET's breakdown voltage, leading to instant failure.
First, I will show the high voltage spikes produced by the transformer switching at the MOSFETs drain. For this I will place a voltage at the MOSFET drain. I will now plot the graph of the signal at the drain.
As you seen on the graph, there is voltage spikes on top of the actual square wave signal waveform. This spike reaches up to 50 to 55V which can damage the IRFZ44N MOSFETs.
Next, I will place voltage probe at the two terminals of the output ac voltage. To plot the output ac signal which is a differential signal Va-Vb, we have to add probe to the graph and configure the signal expression Vout=P1-P2. This is how to plot differential signal. See How to plot frequency spectrum of differential signal in proteus and Differential Measurement using the Oscilloscope Math Function in Proteus. So, this is the 220V AC differential signal at the output of the inverter. As you can see there is no high voltage spikes at the inverter output.
Now I will place the RC snubber circuit across the drain and source of each MOSFETs.
To calculate the resistor R and capacitor C values I will use an online RC snubber calculator. Here I have used output power as 150 Watt, the source battery voltage is 12V. The signal frequency is 50Hz, the leakage inductance is 22uH and the dV, the high voltage tolerance, is 20V.
The theoretical calculate value of resistor(R) is 1.6Ohm and capacitor(C) is 8.59uF. Now, we will use these theoretical values in our RC snubber circuit.
Now, I will restimulate the graph of drain signal.
As you can see the high voltage spikes are gone, that is, reduced to manageable level from 55V to around 25V. Now the MOSFETs power stage is save from high emf voltage from the transformer.
How it Works
The RC snubber manages this energy through two main actions: Energy Storage (The Capacitor): The capacitor acts as a temporary "buffer" or "bucket." When the switch opens, the inductive energy flows into the capacitor instead of creating a high-voltage needle spike. Energy Dissipation (The Resistor): The resistor dampens the "ringing" (oscillations) that occurs between the inductor and capacitor. It converts the excess energy into a small amount of heat, preventing the circuit from vibrating electrically.
To demonstrate the RC snubber circuit role again, I will remove the snubber circuit connection and redo the simulation. As you can see the high voltage appeared again.
Now I will reconnect the snubber circuit. Restimulate the graph again and as you can see the spikes are gone.
Now I will simulate the inverter 220V ac differential output signal with the RC snubber circuit connected. I will again disconnect the snubber circuit to show the difference in the inverter output signal waveform when the snubber circuit is connected and disconnected.
That's it. The demonstration of RC snubber circuit role in inverter circuit is complete. I hope you learned how the snubber circuit works.
Watch the following video that demonstrates how RC snubber circuit works in inverter circuit.
Thanks for reading. See you in the next.
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